This isn't really a hint, so much as an observation of surprisingly nice behavior by Mail in OS X 10.7.

I discovered this after I received an email from a colleague containing a long bulleted list of remarks and questions (dozens of them) about a project we're working on, only a few of which needed answers.

So I replied, inserting my points under his where needed and clicked send. To my surprise, in Mail, set to conversation view, the letter I sent appeared folded in several places, leaving only the questions I had answered visible.

[crarko adds: I don't have a good conversation to test this with at the moment, so please post your results and observations about this in the comments.]